The present invention relates to a drive circuit for a semiconductor laser utilized in an opto-electric data recording/playback apparatus for digital data on optical disc records.
A semiconductor laser is generally utilized as a source of a light beam which is employed for recording and playback of data by an opto-electric data recording/playback apparatus which employs optical disc records as a recording medium. During playback mode operation, the level of output light power from the laser is held constant at a relatively low value (e.g. approximately 1 mW. During recording mode operation, the output light power level from the laser is modulated to produce output light pulses with a power level which is several times that employed during playback operation. This modulation is performed in accordance with a recording signal, which may represent for example data which has been processed by a computer.
The level of output light power from a semiconductor laser usually displays substantial variations in response to operating temperature changes. It is therefore necessary to stabilize the output light power level, e.g. by means of a feedback control system whereby the output light power level from the laser is monitored by a photo-receptive element such as a PIN diode and a signal produced thereby is fed back to control the drive current supplied to the laser.
When such an opto-electric data recording/playback apparatus is utilized to record and playback data processed by a computer, a controller is generally utilized as an interface between the apparatus and the computer. The controller serves to convert data which has been processed by the computer into a recording signal which is used to modulate the laser output light power level. The controller may also generate a write gate signal which controls changeover of the apparatus between recording operation and playback operation. The write gate signal is usually generated by microprogram operation of a CPU within the controller, and transferred through an output port. Specifically, the microprogram operation functions such that when a recording signal is to be supplied from the controller to the opto-electric data recording/playback apparatus, firstly a write gate signal is initiated, the recording signal is then transferred from the controller to the recording/playback apparatus, and following termination of the recording signal the write gate signal is terminated. FIGS. 2A and 2B are timing diagrams to illustrate the above sequence of operations, and respectively show the write gate signal and the recording signal. Due to the fact that the write gate signal is generated by microprogram operation and then transferred through a CPU output port, a certain degree of program execution time is necessary, so that it is difficult to precisely synchronize the start of the recording signal with the start of the write gate signal. As a result there is a time delay of the order of 10 to 20 microseconds between the start of the write gate signal and the start of the recording signal. In addition, the trailing edge of the write gate signal is delayed with respect to the final trailing edge of the recording signal by a similar amount of delay time.
FIG. 1 is a circuit diagram of an example of a prior art drive circuit for a semiconductor laser of a recording/playback apparatus as described above, which is disclosed in U.S. patent application Ser. No. 758,807 of 25th July 1985. Reference numeral 1 denotes a semiconductor laser, and numeral 2 a PIN diode for monitoring a level of light output from the laser 1. Numeral 3 denotes a current-voltage converter circuit for converting a current which flows through PIN diode 2 and represents a level of light produced by laser 1, into an error voltage which is utilized in a control loop for feedback control of the input power supplied to laser 1, i.e. control of the the laser current. Numeral 4 denotes an operational amplifier forming part of the aforementioned control loop, having a noninverting input to which the error voltage is applied. 5a and 5b denote transistors connected in a common-emitter configuration with semiconductor laser 1 connected to receive the collector current of transistor 5b, whereby modulation of the laser current is accomplished by applying a recording signal B between the base terminals of transistors 5a and 5b. Numerals 8 and 9 denote variable resistors which are utilized to set the values of respective control voltages for determining levels of current flow through semiconductor laser 1, and hence levels of light which are emitted by laser 1 during data recording operation and data playback operation respectively. One of these control voltages is selected by a pair of analog gate switches 7a, 7b to be applied to the inverting input of operational amplifier 4, under the control of a write gate signal A. This write gate signal A and the recording signal B are supplied from a controller 11 coupled to a computer 10, which processes the data to be recorded.
The output voltage from operational amplifier 4 is applied to the base of a transistor 6, whose collector is coupled to the common emitters of transistors 5a and 5b, whereby the level of current flow through these transistors, and hence the level of light produced by laser 1, is determined by the operational amplifier output voltage and a resistor 6b connected to the emitter of transistor 6, with transistor 6 and resistor 6b functioning as a current source.
During playback operation, switch 7b is closed and 7a is opened, whereby a control voltage of value determined by variable resistor 9 is applied to operational amplifier 4, while during recording operation switch 7a is closed and 7b is opened so that a control voltage of value determined by variable resistor 8 is applied.
The write gate signal A is initiated (i.e. in this example, goes from a low logic level to a high logic level) at a point in time prior to transfer of the recording signal B from controller 11, as described above and shown in FIG. 2. When the write gate signal is thus initiated, the analog switch 7a is closed and switch 7b is opened, so that the control voltage applied to operational amplifier 4 is switched from the value required during playback to that required during recording. However due to the fact that the recording signal is delayed with respect to the write gate signal, by approximately 10 to 20 microseconds, no light will be emitted during that delay interval. This is illustrated in FIG. 2C, which shows an idealized waveform diagram of the output light from semiconductor laser 1 in response to recording signal B and light gate signal A, with the initial delay between the recording signal and write gate signal being indicated as b. That is to say, during this interval b, in spite of the fact that the write gate signal has switched the control voltage of operational amplifier 4 to that required during recording operation, the recording signal has not yet been supplied to transistors 5a, 5b, so that no light will be emitted by laser 1. No output current will therefore be produced from PIN diode 2 during interval b, so that no error voltage will be applied to operational amplifier 4 from current-voltage converter circuit 3. As a result, the feedback control loop will enter a condition for generation of a maximum level of output light from laser 1. At the end of the delay interval b, the leading pulse c of the recording signal is applied to the base of transistor 5a, whereby modulation of the level of light produced by semiconductor laser 1 is initiated. However since the feedback control loop is in a condition whereby a maximum level of light is emitted by semiconductor laser 1, during a period of time following the start of modulation operation, the leading pulse of the recording signal and several pulses following thereafter will result in excessively high levels of modulated output light being produced. The modulated light output is shown in FIG. 2D, with the initial excessively high levels being indicated by e. These excessive output levels will eventually be reduced to a predetermined recording level, after a time which is determined by the frequency response of the feedback control loop. However due to the fact that excessive light levels are produced by the semiconductor laser in response to the first few pulses of the recording signal, after switching from playback to recording operation is executed, the leading portion of the data which is thus recorded will not be reliable.
Similarly, the error signal produced from PIN diode 2 during the delay time interval d shown in FIG. 2C will result in an excessive surge of output light level from semiconductor laser 1, as indicated by f in FIG. 2D.
These excessive levels of output light from semiconductor laser 1 result from excessive levels of current flow through the laser, which exceed the maximum rated drive current value, and therefore result in a significant reduction of the operating lifetime of semiconductor laser 1.